Egyptian officials say overnight clashes between security forces and supporters of ousted President Mohamed Morsi have left at least seven people dead and 261 others injured.

The fighting broke out late Monday and lasted into early Tuesday, mainly in central Cairo, as Morsi supporters continue to demand his return to power.

Police fired tear gas at the Morsi loyalists, who retaliated by throwing rocks. Demonstrators also blocked Cairo's Ramses Square and a bridge, drawing tear gas from security forces who pushed them back.

Protesters held a sit-in outside the Rabia el-Adawiya Mosque in Cairo's Nasir City, a stronghold of Morsi's Muslim Brotherhood. Leaders of the Islamist group have vowed to escalate their protests.

Other demonstrations took place across Egypt, including in Alexandria.

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U.S. Deputy Secretary of State William Burns met Monday with leaders of the military-installed interim government, which is trying to move ahead with a transition plan.

Burns said the U.S. will not try to impose its model of democracy on Egypt, and that it recognizes only Egyptians can determine their future. He is the first senior U.S. official to visit since the Egyptian army removed Morsi nearly two weeks ago.

The U.S. administration has been criticized both by Morsi supporters and opponents for what each side has perceived as support for the other.

Interim President Adly Mansour has laid a timetable for new elections. The Muslim Brotherhood has said it will not take part in any interim administration.

Morsi has been held at an undisclosed location since his removal, while a number of senior Muslim Brotherhood members have been taken into custody. Authorities have not charged the former president with a crime, but they say they are investigating a series of complaints against him, including spying and wrecking the economy.